The desire to provide high efficiency power plants with low emissions has resulted in an increased emphasis on the use of fuels that are readily available and that are clean burning. Natural gas is an abundant, clean burning fuel with improved emission levels of both nitrogen oxides and particulate matter. The conversion of diesel engines, which inherently have high efficiency as a result of high compression ratios, into natural gas operation for improved emissions levels has been an aspiration of the internal combustion engine industry for a period of time. A known technique for converting diesel engines to natural gas operation is called dual-fuel operation. A type of dual fuel combustion engine known as a High Pressure Direct Injection (HPDI) gas engine has become known in the art. HPDI gas engines burn a large percentage of gaseous fuel, yielding an improvement (over diesel engines) with respect to the emission levels. In addition, HPDI gas engines purport to achieve the same combustion efficiency, power and torque output as state-of-the-art diesel engines. The operational principle underlying HPDI gas engines is that two fuels are injected under pressure into the combustion chamber near the end of the compression stroke. According to one method, a small quantity of “pilot fuel” (typically diesel) is injected into the cylinder immediately followed by a more substantial quantity of gaseous natural gas. The pilot fuel readily ignites at the pressure and temperature within the cylinder at the end of the compression stroke, and the combustion of the pilot fuel initiates the combustion of the natural gas that might otherwise be difficult to ignite.
When transporting natural gas, the most efficient means is to transport it in a liquid state. Liquefied natural gas (“LNG”) takes up only a fraction (about 1/600) of the volume of natural gas in its gaseous state, and may be maintained in its liquid state in cryogenic compartments. LNG is stored in cryogenic compartments either at or slightly above atmospheric pressure. To produce LNG, natural gas is cooled below its boiling point (about −161° C. at ambient pressure). While it is practical to transport LNG because it takes up a fraction of the volume of natural gas in its gaseous state, natural gas is usually required in its gaseous state for combustion. LNG may be converted into its gaseous form by raising the temperature of the LNG. Detection of any leakage of the LNG from cryogenic tanks on mobile machines that use the natural gas to power HPDI engines is important for both safety and fuel efficiency reasons.
One attempt at detecting leaks of fuel gas in a fuel cell system is described in U.S. Pat. No. 7,648,787 (Suematsu et al.), which describes a method where the amount of fuel gas supplied through a fuel gas supply passage and the amount of the fuel gas consumed by a fuel cell are detected. A difference between the amount of supplied fuel gas and the amount of consumed fuel gas is calculated, and then this difference is corrected by subtracting any changes in the amount of fuel gas in any of the fuel gas passages. The method requires calculating any changes in the amount of fuel gas in the fuel gas passages by measuring the pressure of fuel gas in the fuel gas passages and any change in the pressure of the fuel gas in the fuel gas passages. If the corrected difference between the amount of supplied fuel gas and the amount of consumed fuel gas is greater than a threshold value, a leak is detected.
Although the method disclosed by Suematsu et al. may help to detect leaks of fuel gases, the method is complicated and may introduce additional errors in a determination of leaks as a result of having to determine the actual amount of gas consumed, and having to measure changes in pressure of the fuel gases within fuel passages. Additionally, the method is concerned with the leakage of gaseous hydrogen, and therefore may not provide sufficient accuracy for the determination of leakage of LNG from a cryogenic tank.
The disclosed system and method is directed to overcoming one or more of the problems set forth above and/or elsewhere in the prior art.